system: images: Define the platform in image-types.

Running the following command where my-pine.scm contains an operating-system
declaration:

guix system image --image-type=pine64-raw my-pine.scm

returns:

guix system: error: package linux-libre-arm64-generic@5.18.19 does not support
x86_64-linux

That's because there's no platform defined in the pine64-raw image-type, hence
the guix system image tries to build the image for the host architecture.

* gnu/system/images/novena.scm (novena-image-type): Use the armv7-linux platform.
* gnu/system/images/pine64.scm (pine64-image-type): Use the aarch64-linux platform.
* gnu/system/images/pinebook-pro.scm (pinebook-pro-image-type): Use the aarch64-linux platform.
* gnu/system/images/rock64.scm (rock64-image-type): Use the aarch64-linux platform.

gnu/system/images/novena.scm

@@ -53,7 +53,11 @@
 (define novena-image-type
   (image-type
    (name 'novena-raw)
-   (constructor (cut image-with-os (raw-with-offset-disk-image) <>))))
+   (constructor (lambda (os)
+                  (image
+                   (inherit (raw-with-offset-disk-image))
+                   (operating-system os)
+                   (platform armv7-linux))))))
 
 (define novena-barebones-raw-image
   (image

gnu/system/images/pine64.scm

@@ -58,7 +58,11 @@
 (define pine64-image-type
   (image-type
    (name 'pine64-raw)
-   (constructor (cut image-with-os (raw-with-offset-disk-image) <>))))
+   (constructor (lambda (os)
+                  (image
+                   (inherit (raw-with-offset-disk-image))
+                   (operating-system os)
+                   (platform aarch64-linux))))))
 
 (define pine64-barebones-raw-image
   (image

gnu/system/images/pinebook-pro.scm

@@ -58,9 +58,12 @@
 (define pinebook-pro-image-type
   (image-type
    (name 'pinebook-pro-raw)
-   (constructor (cut image-with-os
-                     (raw-with-offset-disk-image (* 9 (expt 2 20))) ;9MiB
-                     <>))))
+   (constructor (lambda (os)
+                  (image
+                   (inherit
+                    (raw-with-offset-disk-image (* 9 (expt 2 20)))) ;9MiB
+                   (operating-system os)
+                   (platform aarch64-linux))))))
 
 (define pinebook-pro-barebones-raw-image
   (image

gnu/system/images/rock64.scm

@@ -54,9 +54,11 @@
 (define rock64-image-type
   (image-type
    (name 'rock64-raw)
-   (constructor (cut image-with-os
-                     (raw-with-offset-disk-image (expt 2 24))
-                     <>))))
+   (constructor (lambda (os)
+                  (image
+                   (inherit (raw-with-offset-disk-image (expt 2 24)))
+                   (operating-system os)
+                   (platform aarch64-linux))))))
 
 (define rock64-barebones-raw-image
   (image